add capacity bins

[oblonski]

Assume there is a motorbike that can load 10 items of A in bags of both sides of the motorbike. The only way to model this is capacity -> [20] which can yield invalid results. For example, if there is a shipment with size -> [15], it can be loaded onto the motorbike, even if it is not allowed in reality. Therefore, one should be able to model something like this: capacity -> [[10],[10]].